Indeed, the season might have topped 19 named storms, but forecasters at the National Hurricane Center in Miami didn't catch a blink-and-you'll-miss-it storm in early September that quickly reached tropical-storm status, only to weaken hours later. It spent its brief life well off the US East Coast.

Forecasters noticed the storm as they performed their usual season's-end review of data on Atlantic-basin activity. The review also led to Tropical Storm Nate's promotion to Hurricane Nate.

Although the number of named storms was well above the long-term average of 11, the number of hurricanes and major hurricanes were only slightly above average, according to the center's post-season analysis.

The storm's winds and heavy rains assaulted a landscape along much of the coast and deep into New England whose trees were laden with leaves and whose roots were clinging to soils already saturated from previous rain storms. Storm-felled trees and limbs left 4 million customers without electricity.

"Irene broke the 'hurricane amnesia' that can develop when so much time lapses between land-falling storms," said National Weather Service director Jack Hayes in a statement.

All along its path from the Caribbean northward, Irene inflicted an estimated $10.1 billion in damage; the storm reportedly killed 56 people.

Tropical storm Arlene and tropical storm Lee added a combined 46 fatalities to a season that would reach $11.6 billion in damages throughout the Atlantic, Caribbean, and Gulf of Mexico, and 120 casualties.

The season also showcased the advances forecasters have made in providing local emergency managers with timely warnings. Still, efforts are underway to extend storm-track forecasts another two days beyond the current 5-day outlook while reducing the uncertainties in the forecast.

In addition, researchers have placed a heavy emphasis on understanding the drivers behind rapid changes in storm intensity. Last-minute shifts in strength, up or down, ahead of landfall can have a profound effect on the extent of coastline that needs evacuation.

The efforts are being driven in no small part by analyses showing that between 1900 and 2005, damage from landfalling tropical cyclones in the US doubles every 10 years. Costs are rising as more people move to vulnerable coastal areas, triggering the construction of homes, factories, office buildings, and other assets needed to sustain them.